fix(legacy-json): misc promotion logic fix

[flakey5]

Re nodejs/node#57343 (comment)

Description

Validation

Related Issues

Check List

• I have read the Contributing Guidelines and made commit messages that follow the guideline.
• I have run node --run test and all tests passed.
• I have check code formatting with node --run format & node --run lint.
• I've covered new added functionality with unit tests if necessary.

[vercel]

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Project	Deployment	Preview	Updated (UTC)
api-docs-tooling	Ready	Preview	Nov 28, 2025 11:37pm

[codecov]

Codecov Report

❌ Patch coverage is 97.50000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.60%. Comparing base (a97df05) to head (bcb615e).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
.../legacy-json/utils/__tests__/buildSection.test.mjs	97.10%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #493      +/-   ##
==========================================
+ Coverage   74.10%   74.60%   +0.50%     
==========================================
  Files         111      112       +1     
  Lines       10562    10700     +138     
  Branches      701      722      +21     
==========================================
+ Hits         7827     7983     +156     
+ Misses       2732     2714      -18     
  Partials        3        3              

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[ovflowd]

@flakey5 could you add unit tests?

[ovflowd]

SGTM! thanks for quickly going through it!!